Is it possible that the tickets in question were created before upgrading to SW 5.3 that contained the merge ticket feature? I had some similar issues when we went to 5.3, and I could not close older tickets as duplicates.

Funny. I ran into that myself today where I had 2 tickets, from the same person, be each one was assigned to a different site. So I was unable to merge and close. There is a feature request to be able to edit the site. Go here to add your vote to it: